1. Field of the Invention
This invention relates to acoustic ink printing and, more particularly, to methods and means for improving the visual quality of images printed from a high-speed acoustic ink printhead by shifting temperature induced visual artifacts to a high spatial frequency beyond the visual acuity of humans. Portions of the ink contained within the printhead are selectively cooled using counter-flowing heat absorbing fluids and images are generated so that adjacent pixels on a page are produced from interlaced opposite transverse hot-to-cold and cold-to-hot ink well portions of the printhead.
2. Description of Related Art
In acoustic ink printing, an array of ejectors forming a printhead is covered by pools of liquid ink. Each ejector selectively directs a beam of sound energy against a free surface of the liquid ink. The impinging acoustic beam exerts radiation pressure against the surface of the liquid. When the radiation pressure is sufficiently high, individual droplets of ink are ejected from the liquid surface to impact upon a target medium, such as a sheet of paper, to complete the printing process.
Typically, the ejectors are arranged in a linear array that is aligned perpendicular to the movement of the recording medium which receives the ejected ink droplets. Alternatively, the ejectors may be arranged in an array of rows and columns, with the rows stretching across the width of the recording medium and the columns of ejectors arranged approximately perpendicular along the movement of the printhead relative to the recording medium. Often, the columns of ejectors are not arranged exactly perpendicular to the ejector rows, but at oblique angles with the rows. In other words, the ejector rows of the array are staggered.
Each ejector for an acoustic ink printer must be supplied with ink and a good ink supply system should maintain a constant flow of ink to the ejectors. A flowing ink supply system cools the ink and stabilizes the ink temperature more easily. Additionally, the flowing ink supply system keeps the ink free of various contaminants, such as paper dust which might settle upon the free surfaces of the ink, by sweeping the contaminants away. The constantly flowing ink also maintains a fresh ink supply to the free surfaces. Without the constant flow of ink, the differing evaporation rates of the constituents within inks that contain volatile components may adversely affect the uniformity of the ink composition associated with each ejector and, therefore, would also affect the uniformity of performance of the ejectors.
Ideally, each ejector when activated ejects an ink droplet identical in size to the droplets of all the other ejectors in the array. Thus, each ejector should operate the same under ideal conditions.
As can be appreciated, the specialized inks used in acoustic printers are sensitive to temperature. As a general rule, ink drop volume increases with temperature, so temperature non-uniformities can lead to unintended variations in the ink density on the receiver medium. The effect of thermally-induced ink volume variations across the face of standard acoustic ink printheads such as may be caused by the constant flow of ink through the printhead is visible to the naked eye. This results in an overall poor quality image on the transfer medium.
As a general rule of thumb, the total drop diameter non-uniformity should be held to within a target value of 5%. That target value is an approximate upper limit to achieve sufficient uniform optical density. However, a temperature difference of 1.3xc2x0 C. across only four centimeters of the acoustic ink printhead can account for as much as 1.6% of the total drop diameter non-uniformity.
The present invention solves or substantially mitigates the problem of total drop diameter non-uniformity due to heat generation and thermal effects that occur in acoustic ink printheads.
The present invention uses at least two counter-flowing heat absorbing fluid flows to selectively cool portions of the ink in acoustic printheads so that the visual effects caused by thermally-induced ink volume variations are shifted to a high spatial frequency on the receiver medium and are thereby masked to the human eye.
In accordance with one aspect of the invention, a device is provided in an acoustic ink printhead for interlacing temperature induced ink drop volume artifacts to a pixel level frequency above the visual acuity of humans, preferably above about 300 dots per inch. The device includes first and second heat sinks on the printhead adapted to develop, respectively a first temperature gradients in first and second ink drops ejected from the printhead. The first temperature gradient is preferably oriented in a first direction transverse to the longitudinal path of the moving printhead. The second temperature gradient is preferably oriented in a second direction opposite the first direction of the first gradient and transverse to the longitudinal path of printhead motion. The printhead draws ink for adjacent pixels marked on the recording medium from ink wells associated with the first and second gradients in an alternating fashion so that temperature induced ink volume variation artifacts are carried or shifted to the pixel level frequency, preferably above 300 dpi. Ink droplet delivery alternates in a spatial direction transverse to the longitudinal path L across the face of the printhead between oversized drops produced from ink adjacent a first one of the heat sinks and undersized drops produced from ink adjacent a second one of the heat sinks.
In accordance with one aspect of the present invention, therefore, an apparatus is provided for cooling ink in an acoustic printhead having a plurality of rows of ink ejectors arranged on the printhead for ejecting a plurality of rows of ink drops as the printhead translates adjacent an ink drop receiving medium in alternate linear first and second translation directions. The apparatus includes a first tank containing a volume of a first thermally conductive fluid and a second tank containing a volume of a second thermally conductive fluid. A first inlet and a first outlet port on the first tank enable a transverse flow of the first thermally conductive fluid through the printhead. Similarly, a second inlet port and a second outlet port on the second tank enable a transverse flow of the second thermally conductive fluid through the printhead. The first tank has a first surface adapted to conduct thermal energy from a first portion of the ink in the printhead and the second tank includes a second surface adapted to conduct thermal energy from a second portion of the ink in the printhead. The first inlet and outlet ports are arranged on the first tank to establish a flow of the first thermally conductive fluid in a first direction transverse of the translation direction of the printhead. The second inlet and outlet ports are arranged on the second tank to establish a second flow of the second thermally conductive fluid in a second direction opposite the first direction and transverse the translation direction of the printhead.
In accordance with still another aspect of the invention, the first and second tanks are adapted to establish substantially equal and opposite thermal gradients in the ink contained within the printhead. The first tank forms a first thermal gradient in the first direction transverse the translation direction of the printhead. The second tank forms a second thermal gradient in the second direction opposite the first direction and transverse the translation direction of the printhead. Preferably, the first and second thermal gradients have substantially identical characteristics. In that way, the visual effects on the receiver medium caused by ink drop size variation occurring across lead rows of the printhead are effectively cancelled or masked by the substantially equal and opposite thermal effects that are created in the trailing ejector rows.
In accordance with yet another aspect of the invention, the first and second thermally conductive fluids are a one or more of diethelyne glycol triethylene glycol, tetraethylene glycol, or glycerol. In another aspect, the first and second thermally conductive fluids are ink pools flowing within the printhead.
In accordance with yet another aspect of the invention, a rigid divider member is disposed on the printhead between the first and second tanks to form a wall therebetween for separating the first flow of the first thermally conductive fluid from the second flow of the second flow of the second thermally conductive fluid and to provide a mechanical rigid support member in the thermal printhead for stiffening the printhead in a direction transverse the translation direction of the printhead.
In accordance with yet another aspect of the invention, the first and second tanks in the printhead containing the first and second volumes of thermally conductive fluid effectively form first and second heat sinks in the printhead that are adapted to develop, respectively, first and second temperature gradients in the ink held within the printhead. The first heat sink is disposed on the printhead at a first location adjacent a first set of rows of ejectors among several linear series of ejectors to develop the first temperature gradient in a first set of ink drops ejected from the first row of ejectors. The second heat sink formed by the second flow of the second thermally conductive fluid is disposed on the printhead at a second location adjacent a second set of rows of ejectors to develop a second temperature gradient in a second set of ink drops ejected from the second row of ejectors.
In accordance with yet a further aspect of the invention, the first and second heat sinks are adapted to respectively generate the first and second gradients at respective first and second levels and within respective first and second ranges so that thermal growth differences between the first and second sets of ink drops when delivered onto the transfer medium are substantially mutually visually offset. Also, the total drop diameter non-uniformity is held to well within the target value of 5% across the printhead.
One benefit of the invention is that the offsetting thermal gradients created within the printhead effectively mask the visual effect of thermally-induced ink drop volume variations.
It is yet another benefit of the invention that the wall formed between the chambers holding the first and second thermally conductive fluids provides a mechanical stiffening to the printhead to improve the mechanical integrity thereof. The addition of the physical separator between ejector rows in the form described substantially reduces the deflection in the printhead to within approximately two microns.
Still yet other advantages and benefits of the invention will become apparent to those skilled in the art upon reading and understanding the following detailed description.